Polycarbonate plastics possess outstanding strength, impact resistance, clarity, and formability. For these reasons, polycarbonate plastics have been used as a substitute for glass in such applications as eyeglass lenses, airplane canopies, and airplane windows. However, polycarbonate is not without disadvantage. It is easily scratched and abraded, particularly by ice, high velocity rains, and sand; it is attacked and crazed by solvents; it is difficult to polish; and it is subject to discoloration, crazing and cracking from ultraviolet light. Therefore, attempts have been made to coat the polycarbonate to protect it from the environment, while taking advantage of polycarbonate's high strength and toughness.
One attempt is described in U.S. Pat. No. 3,810,815 which discloses a transparent laminate formed by diffusion bonding acrylic resin sheet to polycarbonate sheet. The resulting product overcomes many of the disadvantages of polycarbonate. Although the acrylic sheet is relatively soft, scratches on it are easily removed merely by polishing the marred area. In addition, acrylic is fairly stable from a chemical standpoint and it accepts ultraviolet light stabilizers without significantly impairing its clarity.
However, acrylic is crazed by aggressive solvents such as ketones (acetone, methyl ethyl ketone), aromatic hydrocarbons (benzene, toluene), lacquer thinners, paint strippers, and chlorinated solvents at low stress levels. Furthermore uncured acrylic cannot be cast directly onto the polycarbonate sheet because methyl methacrylate monomer crazes and attacks polycarbonate. In addition, it has been found in practice that the combination of acrylic and polycarbonate results in a loss of toughness of the polycarbonate sheet. It is theorized that cracks initiated on the surface of the acrylic propagate through the lamination and result in low impact strength for the acrylic/polycarbonate combination.
To overcome this crack propagation problem, interlayers have been used to insulate the polycarbonate from the deleterious effects of the acrylic. Interlayers typically are made of silicone rubber or polyvinylbutyral. Although interlayers overcome the problem of loss of impact strength of the polycarbonate, the resulting three layer lamination, depending on the choice of interlayer, is difficult and costly to manufacture, cannot be easily formed, has high weight, and has low temperature resistance and low clarity compared to polycarbonate. In addition the dibutyl sebacate plasticizer used in aircraft grade polyvinylbutyral interlayers can craze the polycarbonate.
Another system used to protect polycarbonate from the environment involves coating polycarbonate with organic coatings about 0.0005 inch thick. By using such thin coatings, the coated polycarbonate product can easily be formed into irregular shapes such as airplane canopies. However, these coatings are too thin for incorporation of enough ultraviolet light stabilizers to be effective, and thus the product loses adhesion when exposed to sunlight. Furthermore, exposure to UV radiation and moisture has been shown to be the cause of surface crazing of naturally weathered polycarbonate in areas such as Arizona and Florida. UV light tends to degrade the bond between the protective coating and the polycarbonate and causes the coating to peel off. In addition, the coating is attacked by some solvents. Also, abrasive solids, such as ice and sand, and high velocity rain rapidly scour off the coating. Rain erosion tests show that thin protective coatings are removed in only five minutes when exposed to rain at 500 miles per hour.
Another method of protecting polycarbonate sheet is described by Hudson in U.S. Pat. No. 3,069,287. This patent discloses protecting polycarbonate with thin polyurethane coatings whereby a polycarbonate article is immersed in a sulfuric acid solution of an alkali metal dichromate, the article is dried, and then coated with a polyurethane coating solution. However, the resulting product cannot be formed. Furthermore, solvent in the polyurethane coating solution can attack the polycarbonate during the casting operation. In addition, thin solvent based coatings are removed in seconds when exposed to rain striking the coating at speeds of 600 mph, a common airplane air speed. Also, solvent cast coatings must be very thin or else they adversely affect the optical qualities of the polycarbonate.
Another method of protecting polycarbonate from the ravages of the environment is disclosed in U.S. Pat. No. 3,388,032, issued to Sanders. This patent describes a five layer laminated article consisting of a sheet of polycarbonate sandwiched between two sheets of thermoplastic polyurethane, which in turn are sandwiched between two sheets of glass. However, this product is not suitable for applications where a curved laminate is required because it cannot be formed due to the presence of the glass. Also, glass adversely affected the impact properties of a laminate, since the glass splinters under impact and glass splinters may strike the pilot in case of a bird impact.
Thus, no material is available which has all the properties required for high performance transparencies such as aircraft windows and canopies. It is therefore desirable to provide a product which is transparent, resists chemical attack, has high toughness and impact strength, resists the effects of weathering, including ultraviolet light, rain, ice, and sand, and can be formed to the curvatures required for aircraft windows and canopies.